The objective of this project is to elucidate the relationship between molecular composition and topography arrangements of membrane building blocks with reference to plasma membrane function. Bioelectrogenesis, transport and many metabolic phenomena are based on the proper associations of membrane proteins and lipids. Membrane ecto-enzymes are glycoproteins and require a lipophilic environment for optimal activity. Ecto-phosphoesterhydrolases appear to be a part of a regulatory system which modulates membrane permeability and excitability. Using covalently reacting chemical probes we have been able to achieve selective modifications of membrane ecto-enzymes. Ecto-ATPases and ecto-5'-nucleotidases in particular have been the subject of these investigations. These enzymes are embedded in the lipophillic environment of the plasma membrane, but project their catalytic sites into the extracellular environment. Selective modifications of these enzymes are synthesized de novo and inserted into the membrane. Specific domains of the plasma membrane are exfoliated in the form of microvesicles (exosomes) which contain the ecto-enzymes as markers.